Fuel control systems for gas turbine engines typically include fixed positive displacement fuel pumps. Such pumps deliver fuel at a rate sufficient to satisfy maximum system demands. When operating at less than maximum demand, excess fuel is recirculated back to the pump. This circulation of surplus fuel disadvantageously generates excess heat, and consumes power.
To address these drawbacks of fixed delivery fuel pumps, variable delivery, positive displacement fuel pumps may be used to alter the amount of fuel delivered by the pump in response to changes in demand. One known technique for controlling the amount of delivered fuel involves maintaining a constant pressure drop across a metering valve in a fuel delivery path by varying the pump displacement.
A known fuel delivery system for aircraft gas turbine engines includes a variable delivery, positive displacement fuel pump to supply burn fuel flow to the gas turbine engine and servo flow to the servo systems in the fuel control. To ensure adequate burn fuel flow to the gas turbine engine despite variations in servo flow, e.g., due to servo flow surges resulting from repositioning actuators on the engine, commanding changes in burn flow to the engine, etc., surplus fuel flow (“capacitance”) is provided by the fuel pump. In the known variable delivery, positive displacement fuel pump system, the amount of surplus fuel flow varies as a function of the burn flow to the gas turbine engine and fuel system back pressure. A disadvantage of this type of system is that minimum surplus flow must be met at low system back pressures, resulting in excess surplus flow, and therefore excess heat generation, at high system back pressures.